This invention relates to field electron emission materials, and devices using such materials.
In classical field electron emission, a high electric field of, for example, ≈3xc3x97109 V mxe2x88x921 at the surface of a material reduces the thickness of the surface potential barrier to a point at which electrons can leave the material by quantum mechanical tunnelling. The necessary conditions can be realised using atomically sharp points to concentrate the macroscopic electric field. The field electron emission current can be further increased by using a surface with a low work function. The metrics of field electron emission are described by the well known Fowler-Nordheim equation.
There is considerable prior art relating to tip based emitters, which term describes electron emitters and emitting arrays which utilise field electron emission from sharp points (tips). The main objective of workers in the art has been to place an electrode with an aperture (the gate) less than 1 xcexcm away from each single emitting tip, so that the required high fields can by achieved using applied potentials of 100V or lessxe2x80x94these emitters are termed gated arrays. The first practical realisation of this was described by C A Spindt, working at Stanford Research Institute in California (J. Appl. Phys. 39, 7, pp 3504-3505, (1968)). Spindt""s arrays used molybdenum emitting tips which were produced, using a self masking technique, by vacuum evaporation of metal into cylindrical depressions in a SiO2 layer on a Si substrate.
In the 1970s, an alternative approach to produce similar structures was the use of directionally solidified eutectic alloys (DSE). DSE alloys have one phase in the form of aligned fibres in a matrix of another phase. The matrix can be etched back leaving the fibres protruding. After etching, a gate structure is produced by sequential vacuum evaporation of insulating and conducting layers. The build up of evaporated material on the tips acts as a mask, leaving an annular gap around a protruding fibre.
An important approach is the creation of gated arrays using silicon micro-engineering. Field electron emission displays utilising this technology are being manufactured at the present time, with interest by many organisations world-wide.
Major problems with all tip-based emitting systems are their vulnerability to damage by ion bombardment, ohmic heating at high currents and the catastrophic damage produced by electrical breakdown in the device. Making large area devices is both difficult and costly.
In about 1985, it was discovered that thin films of diamond could be grown on heated substrates from a hydrogen-methane atmosphere, to provide broad area field emittersxe2x80x94that is, field emitters that do not require deliberately engineered tips.
In 1991, it was reported by Wang et al (Electron. Lett., 27, pp 1459-1461 (1991)) that field electron emission current could be obtained from broad area diamond films with electric fields as low as 3 MV mxe2x88x921. This performance is believed by some workers to be due to a combination of the negative electron affinity of the (111) facets of diamond and the high density of localised, accidental graphite inclusions (Xu, Latham and Tzeng: Electron. Lett., 29, pp 1596-159 (1993)) although other explanations are proposed.
Coatings with a high diamond content can now be grown on room temperature substrates using laser ablation and ion beam techniques. However, all such processes utilise expensive capital equipment and the performance of the materials so produced is unpredictable.
S I Diamond in the USA has described a field electron emission display (FED) that uses as the electron source a material that it calls Amorphic Diamond. The diamond coating technology is licensed from the University of Texas. The material is produced by laser ablation of graphite onto a substrate.
From the 1960s onwards another group of workers has been studying the mechanisms associated with electrical breakdown between electrodes in vacuum. It is well known (Latham and Xu, Vacuum, 42, 18, pp 1173-1181 (1991)) that as the voltage between electrodes is increased no current flows until a critical value is reached at which time a small noisy current starts flowing. This current increases both monotonically and stepwise with electric field until another critical value is reached, at which point it triggers an arc. It is generally understood that the key to improving voltage hold-off is the elimination of the sources of these pre-breakdown currents. Current understanding shows that the active sites are either metal-insulator-vacuum (MIV) structures formed by embedded dielectric particles or conducting flakes sitting on insulating patches such as the surface oxide of the metal. In both cases, the current comes from a hot electron process that accelerates the electrons resulting in quasi-thermionic emission over the surface potential barrier. This is well described in the scientific literature e.g. Latham, High Voltage Vacuum Insulation, Academic Press (1995).
FIG. 1a of the accompanying diagrammatic drawings shows one of these situations in which a conducting flake is the source of emission. The flake 203 sits on an insulating layer 202 above a metal substrate 201 and probes the field. This places a high electrical field across the insulating layer formed by for example the surface oxide. This voltage probing has been named the xe2x80x9cantenna effectxe2x80x9d. At a critical field the insulating layer 202 changes its nature and creates an electro-formed conducting channel 204. A proposed energy level diagram for such a channel is shown in FIG. 1b of the accompanying diagrammatic drawings. In this model electrons 212 near the Fermi level 211 in the metal can tunnel from the metal 210 into the insulator 216 and drift in the penetrating field until they are near the surface. The high field 213 in the surface region accelerates the electrons and increases their temperature to xcx9c1000xc2x0 C. It is not known precisely what changes occur in the region of the channel but a key feature must be the neutralisation of the xe2x80x9ctrapsxe2x80x9d 217 that result from defects in the material. The electrons are then emitted quasi-thermionically over the surface potential barrier 215. The physical location of the source of these electrons 205 is shown in FIG. 1a and, whilst a proportion of them will initially be intercepted by the particle, it will eventually charge up to a point at which the net current flow into it is zero.
It is to be appreciated that the emitting sites referred to in this work are unwanted defects, occurring sporadically in small numbers, and the main objective in vacuum insulation work is to avoid them. For example, as a quantitative guide, there may be only a few such emitting sites per cm2, and only one in 103 or 104 visible surface defects will provide such unwanted and unpredictable emission.
Accordingly, the teachings of this work have been adopted by a number of technologies (e.g. particle accelerators) to improve vacuum insulation.
Latham and Mousa (J. Phys. D: Appl. Phys. 19, pp 699-713 (1986)) describe composite metal-insulator tip-based emitters using the above hot electron process and in 1988 S Bajic and R V Latham, (Journal of Physics D Applied Physics, vol. 21 200-204 (1988)), described a composite that created a high density of metal-insulator-metal-insulator-vacuum (MIMIV) emitting sites. The composite had conducting particles dispersed in an epoxy resin. The coating was applied to the surface by standard spin coating techniques.
Much later in 1995 Tuck, Taylor and Latham (GB 2304989) improved the above MIMIV emitter by replacing the epoxy resin with an inorganic insulator that both improved stability and enabled it to be operated in sealed off vacuum devices.
All of the inventions described above rely on hot electron field emission of the type responsible for pre-breakdown currents but, so far, no method has yet been proposed to produce emitters with a plurality of conducting particle MIV emitters in a controlled manner.
Preferred embodiments of the present invention aim to provide cost effective broad area field emitting materials and devices. The materials may be used in devices that include: field electron emission display panels; high power pulse devices such as electron MASERS and gyrotrons; crossed-field microwave tubes such as CFAs; linear beam tubes such as klystrons; flash x-ray tubes; triggered spark gaps and related devices; broad area x-ray sources for sterilisation; vacuum gauges; ion thrusters for space vehicles; particle accelerators; ozonisers; and plasma reactors.
According to a first aspect of the present invention there is provided a method of forming a field electron emission material, comprising the step of disposing on a substrate having an electrically conductive surface a plurality of electrically conductive particles, each with a layer of electrically insulating material disposed either in a first location between said conductive surface and said particle, or in a second location between said particle and the environment in which the field electron emission material is disposed, but not in both of said first and second locations, such that at least some of said particles form electron emission sites at said first or second locations where said electrically insulating material is disposed.
Thus, in preferred embodiments of the invention, an emitter may be formed so that a MIV channel is either at the base or the top of the particle. If the MIV channel is at the base, as in FIG. 1a, the antenna effect enhances the electric field across the channel according to the ratio of particle height normal to the surface and insulator thickness. However, it is equally possible to form a MIV channel on the top of the particle by overcoating a particle in electrical contact with the surface with an insulating layer. In this case the field enhancement is based upon the particle shape. For all reasonable particle shapes, one will typically be limited to a field enhancement factor of approximately ten. The arrangement with the lower channel will usually give the lowest switch-on field. The arrangement with the channel on top can be far more robust and would find application in pulsed power devices where high electric fields and large electrostatic forces are the norm and very high current densities are required.
Preferably the dimension of said particles normal to the surface of the conductor is significantly greater than the thickness of said layer of insulating material.
Preferably, said dimension substantially normal to the surface of said particle is at least 10 times greater than said thickness.
Preferably, said dimension substantially normal to the surface of said particle is at least 100 times greater than each said thickness.
In a preferred example, the thickness of said insulating material may be in the range 10 nm to 100 nm (100 xc3x85 to 1000 xc3x85) and said particle dimension in the range 1 xcexcm to 10 xcexcm.
There may be provided a substantially single layer of said conductive particles each having their dimension substantially normal to the surface in the range 0.1 xcexcm to 400 xcexcm.
Said insulating material may comprise a material other than diamond.
Preferably, said insulating material is an inorganic material.
Preferably, said inorganic insulating material comprises a glass, lead based glass, glass ceramic, melted glass or other glassy material, ceramic, oxide ceramic, oxidised surface, nitride, nitrided surface, boride ceramic, diamond, diamond-like carbon or tetragonal amorphous carbon.
Glassy materials may be formed by processing an organic precursor material (eg heating a polysiloxane) to obtain an inorganic glassy material (eg silica). Other examples are given in the description below.
Each said electrically conductive particle may be substantially symmetrical.
Each said electrically conductive particle may be of substantially rough-hewn cuboid shape.
Each said electrically conductive particle may be of substantially spheroid shape with a textured surface.
A field electron emission material as above may comprise a plurality of said conductive particles, each having a longest dimension and preferentially aligned with their longest dimension substantially normal to the substrate.
A field electron emission material as above may comprise a plurality of conductive particles having a mutual spacing, centre-to-centre, of at least 1.8 times their smallest dimension.
Preferably, each said particle is, or at least some of said particles are, selected from the group comprising metals, semiconductors, electrical conductors, graphite, silicon carbide, tantalum carbide, hafnium carbide, zirconium carbide, boron carbide, titanium diboride, titanium carbide, titanium carbonitride, the Magneli sub-oxides of titanium, semi-conducting silicon, III-V compounds and II-VI compounds.
Most metals, most semiconductors and most electrical conductors are suitable materials.
In the case of emitters with a lower channel, or emitters with a channel on top where the particle is partially covered in said insulating material, each said particle may comprise a gettering material.
Preferably, said surface is coated with said particles by means of an ink containing said particles and said insulating material to form said insulating layer, the properties of said ink being such that said particles have portions which are caused to project from said insulating material, uncoated by the insulating material, as a result of the coating process.
Preferably, said ink is applied to said electrically conductive surface by a printing process.
Said electrically conductive particle(s) and/or inorganic electrically insulating material may be applied to said electrically conductive substrate in a photosensitive binder to permit later patterning.
The insulator component of said ink may be formed by, but not limited to, the step of fusing, sintering or otherwise joining together a mixture of particles or in situ chemical reaction.
The insulating material may then comprise a glass, glass ceramic, ceramic, oxide ceramic, oxide, nitride, boride, diamond, polymer or resin.
Each said electrically conductive particle may comprise a fibre chopped into a length longer than its diameter.
Said particles may be formed by the deposition of a conducting layer upon said insulating layer and its subsequent patterning, either by selective etching or masking, to form isolated islands that function as said particles.
Said particles may be applied to said conductive surface by a spraying process.
Said conductive particles may be formed by depositing a layer that subsequently crazes, or is caused to craze, into substantially electrically isolated raised flakes.
Said conducting layer may be a metal, conducting element or compound, semiconductor or composite.
A method as above may include the step of selectively eliminating field electron emission material from specific areas by removing the particles by etching techniques.
Preferably, the distribution of said sites over the field electron emission material is random.
Said sites may be distributed over the field electron emission material at an average density of at least 102 cmxe2x88x922.
Said sites may be distributed over the field electron emission material at an average density of at least 103 cmxe2x88x922, 104 cmxe2x88x922 or 105 cmxe2x88x922.
Preferably, the distribution of said sites over the field electron emission material is substantially uniform.
The distribution of said sites over the field electron emission material may have a uniformity such that the density of said sites in any circular area of 1 mm diameter does not vary by more than 20% from the average density of distribution of sites for all of the field electron emission material.
Preferably, the distribution of said sites over the field electron emission material when using a circular measurement area of 1 mm in diameter is substantially Binomial or Poisson.
The distribution of said sites over the field electron emission material may have a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 4 xcexcm diameter.
The distribution of said sites over the field electron emission material may have a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 10 xcexcm diameter.
A method as above may include the preliminary step of classifying said particles by passing a liquid containing particles through a settling tank in which particles over a predetermined size settle such that liquid output from said tank contains particles which are less than said predetermined size and which are then coated on said substrate.
The invention extends to a field electron emission material produced by any of the above methods.
According to a further aspect of the present invention, there is provided a field electron emission device comprising a field electron emission material as above, and means for subjecting said material to an electric field in order to cause said material to emit electrons.
A field electron emission device as above may comprise a substrate with an array of emitter patches of said field electron emission material, and control electrodes with aligned arrays of apertures, which electrodes are supported above the emitter patches by insulating layers.
Said apertures may be in the form of slots.
A field electron emission device as above may comprise a plasma reactor, corona discharge device, silent discharge device, ozoniser, an electron source, electron gun, electron device, x-ray tube, vacuum gauge, gas filled device or ion thruster.
The field electron emission material may supply the total current for operation of the device.
The field electron emission material may supply a starting, triggering or priming current for the device.
A field electron emission device as above may comprise a display device.
A field electron emission device as above may comprise a lamp.
Preferably, said lamp is substantially flat.
A field electron emission device as above may comprise an electrode plate supported on insulating spacers in the form of a cross-shaped structure.
The field electron emission material may be applied in patches which are connected in use to an applied cathode voltage via a resistor.
Preferably, said resistor is applied as a resistive pad under each emitting patch.
A respective said resistive pad may be provided under each emitting patch, such that the area of each such resistive pad is greater than that of the respective emitting patch.
Preferably, said emitter material and/or a phosphor is/are disposed upon one or more one-dimensional array of conductive tracks which are arranged to be addressed by electronic driving means so as to produce a scanning illuminated line.
Such a field electron emission device may include said electronic driving means.
The environment may be gaseous, liquid, solid, or a vacuum.
A field electron emission device as above may include a gettering material within the device.
Preferably, said gettering material is affixed to the anode.
Said gettering material may be affixed to the cathode. Where the field electron emission material is arranged in patches, said gettering material may be disposed within said patches.
In one embodiment of the invention, a field emission display device as above may comprise an anode, a cathode, spacer sites on said anode and cathode, spacers located at at least some of said spacer sites to space said anode from said cathode, and said gettering material located on said anode at others of said spacer sites where spacers are not located.
In the context of this specification, the term xe2x80x9cspacer sitexe2x80x9d means a site that is suitable for the location of a spacer to space an anode from a cathode, irrespective of whether a spacer is located at that spacer site.
Preferably, said spacer sites are at a regular or periodic mutual spacing.
In a field electron emission device as above, said cathode may be optically translucent and so arranged in relation to the anode that electrons emitted from the cathode impinge upon the anode to cause electro-luminescence at the anode, which electro-luminescence is visible through the optically translucent cathode.
It will be appreciated that the electrical terms xe2x80x9cconductingxe2x80x9d and xe2x80x9cinsulatingxe2x80x9d can be relative, depending upon the basis of their measurement. Semiconductors have useful conducting properties and, indeed, may be used in the present invention as conducting particles. In the context of this specification, each said conductive particle has an electrical conductivity at least 102 times (and preferably at least 103 or 104 times) that of the insulating material.